Modern wireless telecommunications systems are evolving to provide high speed packet data services for users of mobile equipment. One example is an ability to provide Internet access to a user of mobile equipment. A wireless system that is rapidly evolving in this direction is a Time Division, Multiple Access (TDMA) system known as the Global System for Mobile Communication (GSM), in particular enhanced versions of GSM known as GSM+, GPRS (General Packet Radio Services) and EGPRS (Enhanced General Packet Radio Services).
In the ongoing GSM/GPRS Release '00 (Release 2000) standardization process two significant issues have arisen. First, the GSM EDGE radio access network (GERAN) is specified as a new access network to the 3rd generation Universal Mobile Telecommunication System (UMTS) core network. Second, voice services are also to be provided through the packet switched core network (GPRS). However, voice traffic has a much more stringent delay requirement, both in the terms of absolute delay and delay jitter, than data services. As such, voice traffic is handled differently in the radio interface, i.e., the channel requests and allocations are not made on a voice packet (frame) basis, but instead a channel is allocated to a voice user for an unpredictable period of time. This type of allocation is referred to as either a dedicated mode or as a fixed allocation mode, and it resembles channel allocation in the conventional circuit switched voice mode.
In order to take advantage of the packet network in the radio interface, a “statistical multiplexing” approach has been proposed for GERAN '00. In the proposed statistical multiplexing approach the channel allocated for a first voice user can be reallocated to a second voice user when the first user goes to the Discontinuous Transmission (DTX) mode, i.e., when there are no speech frames to be transmitted or received (typically during pauses in speech). When the user's mobile station is again required to transmit or receive voice frames, a new channel allocation is required. In the uplink direction (mobile station to network) a channel request is required to be sent to indicate the beginning of a new voice period.
However, the inventor has realized that if the current channel is released each time a DTX period occurs, then an inordinate amount of signaling is required for performing the required channel reallocations.
More particularly, in current digital cellular wireless communication systems such as GSM, a channel that is allocated to the mobile station is dedicated to that mobile station for as long as a call is ongoing, whether or not there is voice/data to be transmitted. However, in the proposed statistical multiplexing approach, the same uplink channel or downlink channel may be allocated to another mobile station when the first mobile station is not transmitting or receiving voice or data (i.e., during the DTX period).
Referring to FIG. 1A, the required release and allocation of uplink channels would clearly result in a large amount of signalling overhead. For example, in step 1 the mobile station indicates a start of a DTX period. The start of a DTX period may typically occur during a normal pause in speech. When the mobile station is required to again send voice frames it must request a channel, shown in step 2, and the network allocates a new channel in step 3. The new voice frames are transmitted in step 4. The process is repeated for the next DTX period, as can be seen in steps 5–7, and for each DTX period thereafter.
Referring now to FIG. 1B, a similar situation exists in the downlink direction. In step 1 the network indicates a DTX period to the mobile station. In response, the mobile station releases the currently allocated downlink voice traffic channel and begins monitoring a control channel in order to obtain a new channel allocation. In step 2, at the end of the downlink DTX period, the network sends the new downlink traffic channel allocation to the mobile station, to which the mobile station responds by tuning to the new traffic channel. At step 3 the mobile station begins receiving voice frames. This process can be repeated many times during a call, as indicated at steps 4, 5 and 6.
It can be appreciated that with normal pauses in speech occurring in both the uplink and the downlink directions many times during a typical call, a large amount of additional signalling would be required to implement the statistical multiplexing proposal.
In addition to the increased signalling burden, another perceived problem with this proposed type of statistical multiplexing is that some signal at the beginning of a first talk burst can be clipped after the DTX period ends, while waiting for a new channel to be allocated. This is due primarily to the finite delay required for the channel allocation process to complete. Since physical channels are being allocated to the mobile station, this time delay also includes the time required to retune the transmitter (uplink) or receiver (downlink) to the newly allocated traffic channel, to settle the transmitter or receiver, and to begin transmitting or receiving the next voice frames.
Another perceived disadvantage inherent in the proposed statistical multiplexing approach is that it becomes very difficult or impossible to transmit “comfort noise” (CN) parameters during the DTX period. CN parameters are generated during the DTX period to characterize or estimate the background audio noise, and are periodically transmitted from (or to) the mobile station. The use of the CN parameters helps to avoid unnatural transitions in background noise that may occur during the DTX period, and when going from the DTX period to the next speech period. Reference with regard to the generation of comfort noise parameters and the DTX period in general can be made to commonly assigned U.S. Pat. No. 5,960,389 “Methods for Generating Comfort Noise During Discontinuous Transmission”, by Kari Jarvinen et al., and to commonly assigned U.S. Pat. No. 5,835,889, “Methods and Apparatus for Detecting Hangover Periods in a TDMA Wireless Communication System Using Discontinuous Transmission”, by Pekka Kapanen.
In the instant case, if the mobile station automatically relinquishes the allocated uplink channel when the DTX period begins, then the mobile station loses the ability to transmit the CN parameters to the network. As a result, the perceived voice quality would be degraded. Although an uplink control channel might be used for this purpose, it can be appreciated that placing the additional CN parameters signalling burden on the relatively scarce control channels (as compared to the more numerous traffic channels) is not an optimum solution to this problem. The same disadvantages exist in the downlink direction.
Although discussed above in the context of voice traffic, the statistical multiplexing technique could be used as well for circuit switched data channels, as DTX has been defined, at least in GSM, also for circuit switched data (the CN parameters are, of course, not employed).
As can thus be appreciated from the foregoing discussion, the requirement to provide voice and circuit switched data services in a packet switched wireless network, particularly one that employs statistical multiplexing approaches for placing more than one user on a channel, presents a number of technical challenges that have heretofore not been adequately addressed.